CorrosionDOE-HDBK-1015/1-93SPECIALIZED CORROSIONRev. 0CH-02Page 29Figure 12 Representation of Crevice PittingIn iron that is exposed to water, a similar action can occur if adjacent areas of the metal surfacebecome exposed to solutions with different oxygen concentrations. For example, the solutionin a crevice exchanges slowly with the bulk of the solution outside the crevice. Oxygen in thesolution inside the crevice will be depleted initially by the corrosion reaction.(2-12)This reaction alone does not produce a protective film on the metal. Because of restricted flowinto the crevice, replenishment of oxygen will be very slow; therefore, the solution inside thecrevice will have a low oxygen concentration relative to that outside the crevice as shown inFigure 12. The two adjacent areas then establish a concentration cell with electrons flowingfrom the region of low oxygen concentration to the region of high concentration. Thus, metalgoes into solution (oxidation) inside the crevice, and reduction occurs outside the crevice.Metal ions diffuse out of the crevice, more metal dissolves, and the process continues. Thisresults in the formation of a pit inside the crevice.
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